The phenomenon of compensatory renal growth has been observed in both experimental and clinical situations. Humoral factors which stimulate renal tubular epithelial DNA synthesis in vitro have been identified in New Zealand white rabbit post-nephrectomy serum. Successful isolation and identification of such factors will permit (1) study of the basics of the cellular response to these growth factors and (2) evaluation of the applicability of exogenously administered factors in renal disease states to enhance growth of residual functioning renal tissue. There exist many clinical renal disease states wherein the functioning residual renal tissue is able to support life only when severe dietary restrictions are imposed or when intermittent artificial external renal support is maintained. If the residual functioning renal tissue of those individuals could be expanded by the administration of specific stimulants, the quality of life in those individuals with marginal renal reserve could be improved. The major objectives of the present study have been (1) to isolate and characterize the organ-specific humoral factors, (2) to identify the renal tubular metabolic systems stimulated by these humoral factors, and (3) to determine whether the isolated factors have clinical value in specific renal disease states.